1. Field of the Invention
The present invention relates to a magnet roller, a process for producing the magnet roller and a developing unit using the magnet roller. More particularly, the present invention is concerned with a magnet roller which is lowered in production cost by a curtailment of the amount of a composition for magnets to be used, is suppressed in lowering of magnetic force due to voids, and is well suited for use in a developing mechanism portion and the like in image formation apparatuses; a process for efficiently producing the aforesaid magnet roller; and a developing unit equipped with said magnet roller.
2. Description of the Related Arts
There is known, in the field of electrophographic equipment and electrostatic recording equipment such as copying machinery and printers, a method for visualizing an electrostatic latent image by locating a magnet roller which is formed with a composition for a magnet in a rotating sleeve as a developing roller for the purpose of visualizing an electrostatic latent image on a latent image holding body such as a photosensitive drum and by supplying the surface of the latent image holding body with a magnetic developer (toner) through a so-called jumping phenomenon that allows the toner which is held on the surface of the sleeve to jump over onto the latent image holding body by means of the magnetic characteristics of said magnet roller.
The above-mentioned magnet roller is constituted of a shaft portion and a magnet main body portion composed of a composition for magnets. The types of said magnet rollers include, for instance, (1) integrated shaft type in which the magnet main body portion and the shaft portion are integrally molded with a composition for magnets; (2) one side inserted shaft type in which the magnet main body portion and the shaft portion on one side are integrally molded with a composition for magnets and a shaft is inserted on the other side; (3) both sides inserted shaft type in which the magnet main body portion is molded with a composition for magnets and shafts are respectively inserted on both the sides;and (4) penetrated shaft type in which the magnet main body portion is molded with a composition for magnets and a shaft is penetrated therethrough.
In regard to a process for producing such a magnet roller, there is prevailingly employed a process which comprises molding a magnet main body portion by injection molding or extrusion molding of a composition for magnets in which magnetic powders are mixed with a thermoplastic resin binder composed principally of nylon or polypropylene, by the use of a mold which has generated a magnetic field on the circumference of a cavity thereof to mold the composition into the form of a roller and magnetize to desired magnetic force characteristics.
The process for forming a magnet main body portion by injection molding or extrusion molding by means of a mold in the above-mentioned manner, is involved in such problems that since the magnet main body portion to be obtained is compacted with a composition for magnets, the amount of said composition to be used is unfavorably increased, thus causing inevitably high cost of said main body and besides, the vicinity of the outside periphery thereof is usually liable to contain unfavorable voids, thereby often resulting in deteriorated magnetic force.